A conventional process for the preparation of a polyethylene terephthalate based resin (PET) is characterized as a two stage process: a melt phase process which includes the esterification and polycondensation reactions, and a solid state polymerization process for increasing the molecular weight of the polymer in the solid state rather than in the melt. In a solid state polymerization process, PET is exposed to temperatures of 200-230° C. and a constant counter-current flow of nitrogen through the resin for a significant length of time. In such a conventional process, the molecular weight of the resin is increased in the melt phase up to an It.V. of about 0.55 to 0.65, followed by pelletization, after which the pellets are crystallized, and then solid state polymerized with an optional annealing step after crystallization.
In the melt phase, residual acetaldehyde is formed by degradation reactions occurring at the high temperatures experienced during the last stages of polycondensation. In a conventional process, attempting to further increase the molecular weight at these It.V. levels causes a marked increase in the formation of acetaldehyde. However, elevated temperatures in the melt phase are required to facilitate the polycondensation molecular weight building reactions. Accordingly, the polymer is made only to a low It.V. of about 0.55 to 0.60 dL/g in the melt phase, followed eventually by the further increase in the molecular weight of the polymer in the solid state.
During solid state polymerization, the particles are exposed to a counter-current flow of nitrogen gas to carry off ethylene glycol, water, and/or other condensates generated during polycondensation. The use of nitrogen also minimizes the oxidative degradation of the PET resin at solid stating temperatures. The nitrogen gas also helps safeguard against oxidation of antimony metal in resins containing reduced antimony as a reheat agent. Although the solid state polymerization provides a product with limited degradation products, the process adds a considerable amount of cost (conversion and capital) to the PET manufacturing process.
It would be desirable to eliminate the step of solid state polymerization by the manufacture of a polyester polymer resin in the melt phase having a high It.V. while minimizing the level of residual acetaldehyde, while also providing a crystallized particle to reduce the agglomeration of the particles in dryers feeding extruders for the formation of articles such as preforms and sheet.